Boiler shows “Err 17” and shuts down when multiple zones call for heat

Safety first: Turn off power and allow the boiler to cool. Do not cap or block the pressure relief valve. Hot water under pressure can cause severe injury.

• Check pressure gauge: Normal hot operating pressure should be under 30 psi. If pressure approaches or exceeds 30 psi, the relief valve may open.
• Inspect expansion tank: Locate the diaphragm expansion tank (often an Extrol-style tank). Tap it: the top should sound hollow and the bottom dull. If it feels full of water, it may be waterlogged.
• Check isolation valves: Ensure any valves between the boiler and expansion tank are fully open so the system can expand.
• Verify fill valve setting: The pressure reducing valve should be set around 12 psi cold. If it is stuck open, it can overfill the system.
• Call a professional for tank service: Recharging or replacing an expansion tank and adjusting system pressure is best done by a heating technician.